In a voltage-controlled oscillator (VCO), a capacitor is charged through a constant current source which is controlled by a control voltage and discharges upon reaching a predetermined charging potential, which results in an oscillation of a typically saw-tooth wave signal corresponding in frequency to the control voltage. The frequency of oscillation can be varied over a relatively wide range by changing the input control voltage. Traditional VCO's have used some variation of a comparator to compare the voltage across the capacitor to a target voltage and, once the target voltage is reached, triggering a resetting of the voltage across the capacitor so that the charging cycle repeats itself and oscillates.
Two traditional VCO topology examples are the integrator-reset oscillator and the exponential oscillator.
The exponential oscillator uses the exponential response of the current across the transistor as related to the input voltage across the base-emitter junction. A drawback to this type of oscillator design is temperature drift and stability due to thermal fluctuations. Because the exponential response of the transistor junction is intrinsically related to the temperature of that junction, VCOs that are driven by exponentiators are vulnerable to the effects of temperature drift. Various methods to overcome this temperature dependence have been developed over the last 50 years, most notably using tempco resistors (temperature-compensated) in the exponentiator and summing amplifiers and/or by heating the transistor junctions to a known constant temperature that is hotter than the ambient so that the transistors remain, in theory, at a constant temperature.
These methods, however, have their technical issues and limitations and neither can fully account for all temperature fluctuations and can take long times to warm up and come to true pitch. Another practical drawback to these approaches is the added manufacturing costs associated with highly matched transistor packages or tempco resistors.
An integrator-reset oscillator uses a simple op-amp integrator circuit with a FET or analog switch to short out the integrating capacitor when the target voltage has been reached. These oscillators are typically very easy to put together and require no costly or specialized components.
The advantage of the op-amp integrator is that the temperature dependencies are minimized (capacitors and resistors do exhibit small temperature dependencies but not on the scale discussed above in the exponentiators). These oscillators come to pitch within about 60 seconds and hold their pitch accuracy over a wide range of normal operating conditions.
The disadvantage of the op-amp integrator is that there is no exponentiation: in other words, these oscillators do not use volt-per-octave input control voltage, but rather require the inputs to be in the exponential scale already.
Typically, these oscillators have been used on instruments with a smaller pitch range where the exponential scale is not such a liability.
Traditional modular synthesizers of the early 1960's and 1970's generated control-voltages from a number of analog sources. As music technology evolved, the need for integrating analog synthesizer components such as VCOs into the digital controlled world of music production became an imperative.
Traditionally, control-voltages were created digitally by using a Digital-To-Analog Converter (or DAC). Early on, these DAC's were very expensive, slow and low-resolution generating control-voltages that were highly compromised due to these limitations.
Generating large numbers of control-voltages simultaneously required the use of a high-precision DAC (typically 16-bits of resolution) and a matrix of analog switches feeding this signal into an array of sample-hold circuits further compromising the final control-voltage by reducing the temporal resolution as well as creating switching artifacts such as charge-injection from the analog switches and sample-hold degradation due to leakage.
Another common way to generate voltages from a micro-processor is by use of PWM (pulse-width modulation). Using a digital output from the micro-processor, a series of pulses are generated of varying width. These are fed into a low-pass filter that in essence integrates the pulse widths and transforms them into a voltage that is proportional to the width (or duty-cycle) of the pulse.
PWM has the advantage of being very low-cost to implement and very easy to control via the micro-processor. The disadvantages have always been the resolution and frequency of the PWM clock which are limited by the hardware design of the micro-processor.